Cartilage enables joints to move smoothly. Cartilage consists essentially of highly specialized cells know as chondrocytes, surrounded by a dense extracellular matrix (ECM). In the case of articular cartilage, the tissue is formed primarily from type II collagen, proteoglycans, and water. Fully matured cartilage has a limited capacity for regrowth and repair in response to damage from trauma or degenerative joint disease. Surgical procedures have been developed for replacement of damaged cartilage with cartilage grown in tissue culture. Bioreactors are used to grow cultured cells, e.g., chondrocytes, for use in generating tissue-engineered cartilage.